From the [] = =! [] is true to parse JavaScript’s various painful type conversions

Variable objects and heap memory

Simple types are in the stack and object types are in the heap var a = 20; var b = 'abc'; var c = true; Var e= {m: 20}// The address is assumed to be 0x0012ff7c var e= {m: 20}// A new memory space is assumed to be 0x0012ff8f console.log(e==d); //falseCopy the code

The variables num, BOL, and STR are basic data types, and their values are stored directly on the stack. Obj, Person, and ARr are compound data types, and their reference variables are stored on the stack, pointing to the actual objects stored in the heap. As you can see from the figure above, we can't manipulate the data in the heap directly, that is, we can't manipulate objects directly, but we can manipulate objects through references to objects in the stack, just like we can manipulate a TV with a remote, except that the TV itself has no control buttons.

First let’s look at the precedence of the operator:

Just see here is not gnashing teeth, the fun is still coming, ha ha

! Taking the inverse operator takes precedence over ==, so let’s see first! How is it defined in ECAMScript?

So! [] will end with a Boolean value (this is crucial in relation to the match selection below).

The behavior of comparing the two is in section 11.9.3, so turn to 11.9.3:

In this algorithm, [] is Object,! [] ==! [] ==! [] matches condition 8, so [] == ToNumber(Boolean) is recursively called for comparison.

[] Returns true or false. This is not up to you or me. The ECMAScript specification defines the result as true or false:

[] is an object, so the value of the corresponding Boolean object is true; So! The corresponding Boolean value for [] is false and then compares [] == ToNumber(false)

Look again at the conversion of Number in the ECMAScipt specification

It can be concluded that the comparison at this time is []==0;

ToPrimitive(x) == y if Type(x) is Object and Type(y) is String or Number. Can be compared with the picture above.

ToPrimitive calls toString by default (as in 8.2.8), so ToPrimitice([]) equals the empty string.

The ECMAScript standard defines the ToPrimitice method:

Is not to see this definition, or a face meng force,ToPrimitive this hell what thing? Doesn’t that mean nothing?

JS::ToPrimitive

I have checked the information, and what I want to say can be summarized as follows:

ToPrimitive(obj,preferredType) takes two arguments, the first obj is the converted object, The second preferredType is the type you want to convert to (default null, accept Number or String). When executing ToPrimitive(obj,preferredType) if the second parameter is null and obj is a Date instance, In other cases, the preferredType is set to Number. If the preferredType is Number, ToPrimitive executes as follows: 1. If obj is the original value, return it; 2. Otherwise, call obj.valueof () and return the original value if the result is executed. 3. Otherwise, call obj.toString() and return it if the result is original; 4. Otherwise, an exception is thrown. If the preferredType is String, swap steps 2 and 3 above, i.e. If obj is the original value, return it; 2. Otherwise, call obj.toString() and return it if the result is original; 3. Otherwise, call obj.valueof () and return the original value if the result is executed. 4. Otherwise, an exception is thrown.Copy the code

Obj.valueof () = obj.tostring () = obj.tostring () = obj.tostring ();

ToString is used to return a string representation of an object.

var obj = {}; console.log(obj.toString()); //[object Object] var date = new Date(); console.log(date.toString()); //Sun Feb 28 2016 13:40:36 GMT+0800Copy the code

The valueOf method returns the original valueOf an object, which may be a string, value, bool, etc., depending on the object.

var obj = { name: "obj" }; console.log(obj.valueOf()); //Object {name: "obj"} var arr1 = [1]; console.log(arr1.valueOf()); //[1] var arr2 = []; console.log(arr2.valueOf()); Var date = new date (); console.log(date.valueOf()); //1456638436303 As shown in the code, three different object instances called valueOf return different dataCopy the code

Primitive values refer to one of the five basic data types [‘Null’,’Undefined’,’String’,’Boolean’,’Number’].

With that in mind, here’s a simple example:

var a={}; ToPrimitive(a) indicates a preferredType, but does not indicate a Date instance, so the preferredType is Number Returns the string string with a.tostring (), the original value. So the final ToPrimitive(a) is "[object object]".Copy the code

If the description is hard to understand, a lot of descriptions are obscure and difficult to understand, we use code to speak:

const toPrimitive = (obj, preferredType='Number') => { let Utils = { typeOf: function(obj) { return Object.prototype.toString.call(obj).slice(8, -1); }, isPrimitive: function(obj) { let types = ['Null', 'String', 'Boolean', 'Undefined', 'Number']; return types.indexOf(this.typeOf(obj)) ! = = 1; }}; if (Utils.isPrimitive(obj)) { return obj; } preferredType = (preferredType === 'String' || Utils.typeOf(obj) === 'Date') ? 'String' : 'Number'; if (preferredType === 'Number') { if (Utils.isPrimitive(obj.valueOf())) { return obj.valueOf() }; if (Utils.isPrimitive(obj.toString())) { return obj.toString() }; } else { if (Utils.isPrimitive(obj.toString())) { return obj.toString() }; if (Utils.isPrimitive(obj.valueOf())) { return obj.valueOf() }; } } var a={}; ToPrimitive(a); //"[object object]Copy the code

ToPrimitive([])= 0; ToPrimitive([])= 0;

So it’s going to be zero“” = = 0In this state, I’m going to keep looking and comparing this picture

Typeof (“”)= string; toNumber(“”)==0; toNumber(“”)==0;

So toNumber(“”)=0, and then the problem of 0 == 0, so []==! [] The answer to the 0 == 0 question is clearly true, with many twists and turns

== null; undefined == null; true = null; And both of them are false when compared to all other values. String == Boolean, both operands need to be converted to Number. 3. String/Boolean == Number, String/Boolean is converted to Number. 4. Object == Primitive. You need to convert Object to Primitive(valueOf and toString). See, there are only four rules! Is it clear and simple?Copy the code

Through a simple graph, let you thoroughly, forever understand JS == operation

What is the conversion priority of the plus operator in JavaScript?

Like my summary of the article point a star my github blog address, summary of the first article, bad and reference do not get support also hope forgive me!